JPH02154467A - Semiconductor device - Google Patents

Abstract

PURPOSE:To decrease only an OFF-state current without decreasing an ON-state current by connecting two or more thin film transistors of different threshold voltage in series and making a gate electrode common to all the transistors. CONSTITUTION:Two or more thin film transistors T1-Tn of different threshold voltage are connected in series, electrodes on both the ends of the thin film transistors T1-Tn connected in series are made a source electrode S and a drain electrode D, and a gate electrode G common to all the thin film transistors T1-Tn connected in series is made. Thereby only an OFF-state current can be decreased without decreasing an ON-state current and gate voltage can be decreased over a wide range.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a thin film transistor (hereinafter abbreviated as TPT) in which a plurality of TPTs are used in order to obtain certain electrical characteristics.
Regarding how to connect.

[Prior Art] Usually, efforts to obtain desired electrical characteristics in a single TPT focus on the structure and manufacturing conditions of the TPT. However, even if the structure and manufacturing conditions of the TPT are devised,
Desired electrical characteristics may not be obtained with only one TPT. In the above case, TPTs having electrically the same basic characteristics were conventionally connected in series or in parallel.

For example, when a high-voltage TPT is required for use at high voltage, a basic TPT with the same basic electrical characteristics is required.
The PTs were connected in series. Furthermore, when a large current was required, basic TPTs having the same basic electrical characteristics were connected in parallel.

Furthermore, when it is desired to reduce the leakage current of TPTs, basic TPTs having electrically identical basic characteristics are connected in series.

[Problems to be Solved by the Invention] Liquid crystal panels are examples of products to which TPT is applied. Several-inch LCD panels using TPT have already been mass-produced, and even larger panels are being researched.

Research is also being conducted into improving display quality such as the resolution of the panel. However, since TPT is usually formed using amorphous Si on a glass substrate, its electrical characteristics are not necessarily sufficient for displaying large-sized liquid crystal panels or improving the display quality of liquid crystal panels. There wasn't. T
This is because PT uses amorphous Si, so its on-current is essentially small and its off-current (leakage current) is large.

In particular, the off-state current required for the TPT needs to be a low value over a wide range of gate voltages due to the characteristics of the driving method of the liquid crystal panel. For example, in an N-channel type TPT, a low off-state current is required when the gate voltage ranges from Ov to about -1 Ov.

However, TPTs formed using amorphous Si have a problem in that off-state current increases as the gate voltage increases in the negative direction. This is because the leakage current of the PN junction formed on the drain side increases. The increase in the off-state current lowers the voltage of the data signal written from the TPT to the pixels of the liquid crystal panel, thereby degrading the display quality of the liquid crystal panel. In order to lower the TPT's off-state current, the basic TPT with the same basic electrical characteristics has been
There was a method of connecting F'r in series. However, when a plurality of TFTs having electrically identical basic characteristics are connected in series, the on-current is definitely lower than the on-current of one TFT having the basic characteristics.

Therefore, an object of the present invention is to provide a TP with low off-current over a wide range of gate voltages without reducing on-current.
It is to provide T.

[Means for Solving the Problems] Means for solving the problems in the present invention are as follows: (1) A thin film transistor including a source electrode, a drain electrode, and a gate electrode has different threshold voltages.
Two or more of the thin film transistors are connected in series, electrodes at both ends of the thin film transistors connected in series are used as a source electrode and a drain electrode, and gate electrodes of all the thin film transistors connected in series are common. Features.

[Examples] The details of the present invention will be explained below using Examples. FIG. 1 shows an embodiment according to the invention. In Figure 1, T-work, T
2. ...Tn is n TPTs connected in series. Both ends of the n TPTs connected in series are:
They serve as the source and drain, respectively. The gates of the n TPTs are all common. Therefore, the n T and F T can be regarded as one TPT as a whole. Among the n TPTs, at least two TPTs have different Vths.

For simplicity, the electrical characteristics of the TPT, which can be regarded as one TPT as a whole, will be described below, taking as an example a case in which two N-channel TPTs are connected in series.

Figure 2 shows two T P T (T 1
, T x ) are connected in series. This is the same as in FIG. 1 in that the TPTs connected in series have a source and a drain at both ends, and have a common gate. FIG. 3 shows the electrical characteristics of the two TFTs, T i and T2, and the overall electrical characteristics (T) when the two TPTs are connected in series. In FIG. 3, the Vths of T1 and T2 are different. The gate voltage at which the drain current is minimum is T, approximately -5V, and "I"
2 is 0■. In both TPTs, when the gate voltage becomes more negative than the gate voltage at which the drain current becomes minimum, the drain current increases. When the gate voltage increases in the negative direction, the channel portion becomes P type and the concentration of the P wall region increases, resulting in a PN formed by the P wall region and the drain, which is an N type region.
This is because leakage current through the junction increases. This tendency for the drain electrode to increase when the gate voltage increases in the negative direction is undesirable, especially when the TFT is used as a pixel driving transistor of a liquid crystal panel. As the gate voltage increases in the negative direction during the data retention period, the voltage of the data signal written to the pixel electrode via the TPT decreases due to the leakage current, and as a result, the display quality of the liquid crystal panel decreases. This is because it decreases. The thick solid line (T) shown in FIG. 3 shows the overall electrical characteristics when the two TPTs T1 and T2 are connected in series. Since the off-state current is determined by the lower value of the two TPTs, it is almost constant in the gate voltage range of 0 to -1OV, and its value is determined by the lower value of the two TPTs.
lower than the minimum off-state current of each T. On the other hand, the on-current of the TPTs connected in series is similarly determined by the on-current of the lower one of the two TPTs, but since the on-current of T is very large, it is almost equal to the on-current of T2. The value will be the same. That is, by connecting two TPTs in series,
Only the off-state current can be reduced without reducing the on-state current, and can be made low over a wide range of gate voltages. Although the above explanation has been given for the case where two TPTs are connected in series, the same idea can be applied to three or more TPTs.

[Effects of the Invention] According to the present invention, only the off-state current can be reduced without reducing the on-state current, and the off-state current can be made low over a wide range of gate voltages.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing n TPTs connected in series according to the present invention. FIG. 2 is a diagram in which two TPTs are connected in series to explain the present invention in an easy-to-understand manner. FIG. 3 is a diagram showing the electrical characteristics of TPT. S... Source D... Drain G... Gate agent Masataka Kamiyanagi (and 1 other person) (V>'Is@/- and smell

Claims (2)

[Claims] (1) In a thin film transistor equipped with a source electrode, a drain electrode, and a gate electrode, the threshold voltages are different from each other.
Two or more of the thin film transistors are connected in series, electrodes at both ends of the thin film transistors connected in series are used as a source electrode and a drain electrode, and gate electrodes of all the thin film transistors connected in series are common. Characteristic semiconductor devices. (2) The semiconductor device according to claim 1, characterized in that amorphous Si is used.